This invention relates to a control loop in which an input signal is converted, by multiplication by an integrator value, to an output signal that exhibits on average a constant reference value.
Such control loops are used, for example, as so-called AGC (automatic gain control) circuits for the automatic gain adaptation of digital signals. In automatic gain control, from an input signal, an output signal having a constant time-average value, whose level is as a rule adjustable and independent of the level of the input signal, is generated.
In detector circuits for high-frequency signals, the AGC is used to furnish, for further signal processing, an intermediate signal whose time-average level is constant, independently of the field strength of the detected signal of the input stage.
A known control loop for digital signals is illustrated in FIG. 1. The energy of an output signal OUT exhibits on average a constant value that can be set via a reference value REF. High-energy input signals thus must be diminished in amplitude and low-energy input signals must be augmented in amplitude.
The output signal OUT is first adapted in a signal converter 1 in such a fashion that it can be compared to the reference value REF. If the output signal OUT is, for example, an electric current, then a voltage proportional to the current is derived to the signal converter and then compared to a reference voltage. The signal converter 1 can also adapt the output signal OUT in such a fashion that the control behavior of the control loop is as favorable as possible. Signal-processing functions such as for example magnitude formation, squaring, or calculation of the distortion factor are suitable for this purpose. If one of the cited signal-processing functions is selected, it is guaranteed that only positive values are employed for the comparison even in case of a negative output signal OUT.
The comparison itself includes taking a difference with a difference element 2, in which the adapted output signal OUT is subtracted from the reference value REF. The difference element 2 supplies a difference ΔIN. The difference ΔIN is fed to an integrator element 3, which determines an integrator value IW therefrom. The integrator element 3 cumulates the difference ΔIN. If the difference ΔIN is positive, the integrator value IW is increased; if the difference ΔIN is negative, it is decreased. In a multiplication element 4, the input signal IN is multiplied by the integrator value IW. The result of this multiplication is the output signal OUT.
The circuit of FIG. 1 can be described mathematically byΔIN=REF−IN*IW
In the steady-state condition, that is, when the control loop has built up to a steady state, ΔIN=0. Hence it follows thatΔIN=REF−IN*IW=0or, after manipulation,IW=REF/IN
Hence, finally, one obtains the desired output value OUT asOUT=IN*IW=IN*REF/IN=REF
The disadvantage of this circuit is the long buildup time that is required if the energy of the input signal IN deviates substantially from the reference value REF. In case of a large deviation, several hundred cycles are required to set the reference value REF exactly. This long buildup time is not acceptable in time-critical control tasks.
It is a goal of the present invention to identify a control loop for the conversion of an input signal, by multiplication by an integrator value, into an output signal that exhibits on average a constant reference value, which control loop exhibits improved control behavior as a consequence of a simple supplementary circuit.